Oxidation reactions are essential for survival, but excessive oxidation (oxidative stress) is implicated in numerous disease processes, including atherosclerosis, diabetes, ischemic-reperfusion injury, and inflammatory disease, such as rheumatoid arthritis. The free radical theory of aging also hypothesizes that aging result from cumulative oxidative damage to biomolecules. Lipids, especially polyunsaturated lipids in lipo-proteins in blood plasma and cellular membranes, are ready substrates for adventitious oxidation reactions, leading to formation of peroxide derivatives, which undergo fragmentation to yield a broad range of reactive intermediates, including alkanals, alkenals, hydroxyalkenals and malondialdehyde. These compounds may react with nucleophilic groups in protein, resulting in modification of the protein and accumulation of chemical damage in long-lived, slowly turned-over proteins. Lipofuscin, the age-pigment which accumulates in post-mitotic cells, is thought to be the indigestible remnant of reaction of lipid peroxidative damage to proteins in vivo is thought to contribute to the chemical modification and gradual deterioration in the structure and function of proteins with age and to the development of pathology in diseases in which oxidative stress is implicated. At present there are not specific chemical biomarkers for damage to proteins induced by lipid peroxidation reactions. Thus, to understand the role of lipid peroxidation in aging and disease, we plan to identify specific products formed on reaction of peroxidize lipids with protein, compounds termed lipoxidation products, and to develop assays for quantifying these compounds in tissue proteins such as plasma lipoproteins, red cell membrane proteins and skin collagen with age, and in the age pigment, lipofuscin, in arterial plaque lipoproteins and in urine. We have prepared two lysine residues, and describe an assay procedure for their detection in lipoxidize protein by gas chromatography -mass spectrometry. We have also identified several other products characteristic of lipoxidation reactions. Our long-term plan is to measure these and other lipoxidation products in tissue proteins under normal conditions, with increasing age, during disease states, and in response to therapeutic intervention. These studies should provide a clearer insight into the role of lipid peroxidation reactions in oxidative stress and the pathogenesis of disease and in aging.